build: support reproducible builds with consistent man pages

Use a consistent date in the generated man pages
even if SOURCE_DATE_EPOCH is not set.

* Makefile.am: Generate .timestamp in the tarball.
* man/local.mk: Pass .timestamp if available to help2man.
* man/dummy-man: Don't bother with the year in the stub.

Reported in http://bugs.debian.org/806321

man/dummy-man

@@ -48,11 +48,10 @@ inadequate on this system.  Creating a stub man page instead.
 END
 
 progname=`basename_ "$1"`
-year=`LC_ALL=C date +%Y`
 bs='\'
 
 cat >"$output" <<END
-.TH "$progname" 1 "$year" "$source" "User Commands"
+.TH "$progname" 1 "$source" "User Commands"
 .SH NAME
 $progname $bs- a $source program
 .SH DESCRIPTION

man/local.mk

@@ -184,7 +184,8 @@ endif
 	  && $(MKDIR_P) $$t						\
 	  && (cd $$t && $(LN_S) '$(abs_top_builddir)/src/'$$prog$(EXEEXT) \
 				$$argv$(EXEEXT))			\
-	  && $(run_help2man)						\
+	&& : $${SOURCE_DATE_EPOCH=`cat $(srcdir)/.timestamp 2>/dev/null || :`} \
+	&& export SOURCE_DATE_EPOCH && $(run_help2man)			\
 		     --source='$(PACKAGE_STRING)'			\
 		     --include=$(srcdir)/man/$$name.x			\
 		     --output=$$t/$$name.1				\